This is an improvement in the .alpha.-halogenation of aldehydes and particularly in the .alpha.-halogenation of certain aldehydes to form intermediates useful in the production of nematocides and pesticides by a process as described in U.S. Pat. Nos. 3,931,331 and 3,873,624 to Mathew et al. Briefly, the above patents describe a process for preparing an .alpha.-formyl sulfide comprising reacting under reflux conditions a halogen which is chlorine, bromine or iodine with an aldehyde of the formula ##STR3## to form an .alpha.-haloaldehyde of the formula ##STR4## The .alpha.-haloaldehyde is subsequently reacted with a thiol salt of the formula R''' SM to form an .alpha.-formyl sulfide of the formula ##STR5## The .alpha.-formyl sulfide may then be oximated to form a 2-hydrocarbylthioaldoxime of the formula ##STR6## Exemplary of the process is (a) the chlorination with chlorine gas of isobutyraldehyde (hereinafter called IBA) to .alpha.-chloroisobutyraldehyde (hereinafter called CIBA); (b) the thiomethylation of CIBA with sodium thiomethylate to 2-methyl-2-methylthioisopropionaldehyde (MTIBA), (c) the oximation with hydroxylamine sulfate and ammonia to 2-methyl-2-methylthiopropionaldoxime (aldicarb oxime or ADO). These two patents are incorporated herein by reference for a fuller discussion of the process including an identification of R', R", R''' and M.
The present invention is directly concerned with the first step of the above sequence for certain aldehydes, but also relates to subsequent steps insofar as the improvement in the first step provides a better quality reactant therefore and improves the overall yield. In particular, the product of the improved process of the present invention contains a reduced impurity level of homopolymers of the aldehyde and copolymers of the aldehyde and the .alpha.-haloaldehyde. The deleterious effects of these impurities was not hitherto realized. With the preferred conversion of IBA to CIBA, the level of the cyclic homopolymer impurity (IBA).sub.3 and the cyclic copolymers (IBA).sub.2 (CIBA).sub.1 and (IBA).sub.1 (CIBA).sub.2 particularly are reduced. Such oligomers, as well as homopolymers of .alpha.-haloaldehydes such as (CIBA).sub.3 (the latter being recognized in the above patents), are detrimental in subsequent steps because they are generally inert to the thiomethylation. Accordingly, the yield of .alpha.-formyl sulfide such as MTIBA is reduced. Furthermore, the oligomers break down during purification of the MTIBA as by distillation to form free IBA and CIBA. The free CIBA in turn is hydrolyzed during the oximation step to produce free chloride which then can cause corrosion of the stainless steel equipment preferrably used for oximation and subsequent stripping, disposal and distillation steps.